1. Field of the Invention
This invention relates generally to hydrocarbon conversion processes. More specifically, this invention relates to the catalytic isomerization of paraffinic hydrocarbons and the separation facilities for obtaining the feed fraction and recovering the product from the isomerization unit. In addition, this invention relates to combination processes for the isomerization and reforming of naphtha boiling range hydrocarbons wherein such processes are performed to produce naphtha boiling range products having sufficient octane number for use as unleaded motor fuel.
2. Description of the Prior Art
High octane gasoline is required for modern gasoline engines. Formerly it was common to accomplish octane number improvement by the use of various lead-containing additives. As lead is phased out of gasoline for environmental reasons, it has become increasingly necessary to rearrange the structure of the hydrocarbons used in gasoline blending in order achieve higher octane ratings. Catalytic reforming and catalytic isomerization are two widely used processes for this upgrading.
A gasoline blending pool normally includes C.sub.4 and heavier hydrocarbons having boiling points of less than 205.degree. C. at atmospheric pressure. This range of hydrocarbon includes C.sub.4 -C.sub.6 paraffins and especially the C.sub.5 normal paraffins which have relatively low octane numbers. The C.sub.4 -C.sub.6 hydrocarbons have the greatest susceptibility to octane improvement by lead addition and were formerly upgraded in this manner. Octane improvement can also be obtained by rearranging the structure of the paraffinic hydrocarbons into branched-chained paraffins or aromatic compound by isomerization. The C.sub.6 and heavier hydrocarbons can be upgraded into aromatics through catalytic reforming. C.sub.5 hydrocarbons are not readily converted into aromatics, therefore, the common practice has been to isomerize these lighter hydrocarbons into the branch chain isoparaffins. Although the C.sub.6 paraffins can be converted to aromatic hydrocarbons through the hydrocyclization, that conversion also causes a reduction in liquid volume yields. The reduction in liquid volume yields results from increased gas production and conversion to higher density species. Therefore, it is also common practice to charge C.sub.6 paraffins to an isomerization unit to obtain C.sub.6 isoparaffin hydrocarbons. Consequently, octane upgrading commonly uses isomerization to convert C.sub.6 and lighter boiling hydrocarbons and reforming to convert C.sub.7 -plus and higher boiling hydrocarbons.
The combination processes using isomerization and reforming to convert naphtha range feedstocks are well known. U.S. Pat. No. 4,457,832 uses reforming and isomerization in combination to upgrade a naphtha feedstock by first reforming the feedstock, separating a C.sub.5 -C.sub.6 paraffin fraction from the reformate product, isomerizing the C.sub.5 -C.sub.6 fraction to upgrade the octane number of these components and recovering a C.sub.5 -C.sub.6 isomerate liquid which may be blended with the reformate product. U.S. Pat. Nos. 4,181,599 and 3,761,392 show a combination isomerization-reforming process where a full range naphtha boiling feedstock enters a first distillation zone which splits the feedstock into a lighter fraction which enters an isomerization zone and a heavier fraction that is charged as feed to a reforming zone. In both the '392 and '599 patents, reformate from one or more reforming zones undergoes additional separation and conversion, the separation including possible aromatics recovery, which results in additional C.sub.5 -C.sub.6 hydrocarbons being charged to the isomerization zone.
It is also known in the art that further octane enhancement can be obtained by recycling at least a portion of the normal paraffins in the effluent of the isomerization zone back through the isomerization zone to obtain additional conversion of paraffins to isoparaffins. Separation facilities and flow schemes for recycling C.sub.5 paraffins, C.sub.6 paraffins or both through an isomerization unit are shown and described at pages 5-49 through 5-51 of The Handbook of Petroleum Refining Processes edited by Robert A. Meyers, published by McGraw Hill Book Company (1986). Recycling is particularly effective due to the equilibrium nature of the pentane and hexane isomerization reactions.
Schemes for recycling the effluent from an isomerization zone include return of at least a portion of the isomerization effluent to the separation facilities for initially splitting a straight-run naphtha feed into light and heavy fractions for the isomerization and reforming zone, respectively. U.S. Pat. No. 3,018,244 shows such an arrangement where a pentane fraction is recycled and combined with the fresh feed entering a series of fractionation columns for removing light components from the feed and separating the feed into light and heavy fractions for the isomerization and reforming sections. U.S. Pat. No. 2,946,736 shows a process flow scheme for an isomerization-reforming combination where at least a portion of the isomerization zone effluent is combined with a hydrotreated naphtha feed and the reforming zone effluent which then enters a fractionation column for splitting the entering components into light and heavy fractions. The light fraction then undergoes further separation to remove isoparaffins and higher octane components from the normal paraffin hydrocarbons which are charged as feed to isomerization zone.
When the use of lead additives was readily permitted, C.sub.5 and C.sub.6 paraffinic hydrocarbons were the most susceptible to octane improvement by the addition of lead additives. Since these additives are relatively cheap, there was no economic incentive for enhancing the octane number of C.sub.5 and C.sub.6 paraffins through isomerization. As a result, a large number of reforming facilities are in existence that have no isomerization zone or capability for recycling normal C.sub.5 and C.sub.6 paraffins to upgrade the octane value of these components, but contain only the reformer and a splitter section for separating a naphtha boiling range feed into light and heavy components. Consequently, it is highly desirable to provide a method for upgrading the C.sub.5 and C.sub.6 normal paraffins using existing separation facilities to the greatest degree possible.